I asked my dentist the radiation dose of a CBCT scan and the machine gives the data in mGy/cm2.

I was rather used to interpret radiation doses in micro-Sivert because this unit takes into account the sensibility of the body tissues and not only the radiation per se.

A decade ago, all radiation doses in medecine was given in Sivert as far as I know : have there been any changes meanwhile that allow CBCT scan doses to be given in mGy/cm2 ?

If so, why such changes ?

How to convert Gy/cm2 into Sivert for mouth tissues (I asked some dentists and physicists, but they didn't know, and googling it was not much help)?

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    I'm used to the Sivert system, or, formerly, rems. What does the G even stand for? Grams? Milligrams of a particular particle of radiation? Doesn't answer your question, but the way radiation is expressed depends on the source (machine vs. radioactive substance which is measured in curies), dose vs. absorbed particles, etc. Add to that the US standards and the International standards are not equally expressed. Provide links and quotes to your google papers and I'll give this a go, but you must provide concrete evidence of a search. Commented Aug 5, 2023 at 14:18
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    G means Gray, I edited my post to correct that. For example, I found the following link, but I am not quite sure if it is reliable : could you confirm please ? dosewizard.com/2019/08/…
    – huurd
    Commented Aug 5, 2023 at 17:37
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    By the way, what does PA and AP (head PA, shoulder AP) stand for in that link ?
    – huurd
    Commented Aug 5, 2023 at 18:13
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    Please note that your link does not provide a quote or evince the requested search. Commented Aug 5, 2023 at 19:17
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    In radiology, PA usually means the xray was taken posterior to anterior (i.e. CXRs are usually taken with the anterior chest facing the plate, so the radiation hits the back (posterior aspect of the body first) and travels the anteriorly to the plate, and AP is the opposite. Standard films have an orientation: PA, AP, L (lateral or left), D (decubitus), etc. Ordering a CXR will get you a PA and L , unless it's at the bedside, in that case, it's ordered AP CXR (the plate is placed behind the patient.) Commented Aug 5, 2023 at 19:22

1 Answer 1


The units might actually have been mGy•cm^2 (rather than mGy/cm^2). This is the dose (in Grays) multiplied by the area it is administered over and is known as the Dose Area Product. This is used as it correlates well with the actual dose delivered to the patient and can easily be measured (or closely approximated) using an ionisation chamber in the path of the beam. For example, a 5x5cm x-ray field with an entrance dose of 1 mGy will have a DAP of 25 mGy•cm^2.

The Gray (Gy) is a (relatively) new international system (SI) unit of radiation dose, expressed as absorbed energy per unit mass of tissue.

The SI unit "gray" has replaced the older "rad" designation. 1 Gy = 1 Joule/kilogram = 100 rad.

The gray is useful clinically and is the main unit used when planning radiotherapy in radiation oncology (a total dose is given in Grays, and administered in multiple fractions to mimimise toxicity.

Although the gray can be used for any type of ionising radiation, it does not account for biological effects. This is where sieverts are used. The dose in gray is multiplied by a factor depending on the radiation.

I found this converter to convert DAP in Gy•cm^2 into sieverts, based on the type of x-ray examination, though it does not have dental x-rays!

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    Yes indeed, you're right the unit was not written properly. Anyway, thank you for the answer, I deduce I will have to ask again my dentist in order to get the parameters for mouth tissues.
    – huurd
    Commented Aug 10, 2023 at 12:34
  • @huurd I’m still looking for dental x-ray specifics! :)
    – Chris
    Commented Aug 10, 2023 at 14:58
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    Thank you, great !
    – huurd
    Commented Aug 10, 2023 at 15:11

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